Convert HTML string to Slack's own markup language (https://slack.zendesk.com/hc/en-us/articles/202288908-Formatting-your-messages)

html-to-slack.php

<?php

/* Strip out un-supported HTML tags */
$content = strip_tags($content, '<strong><em><del><li><code><pre>');

/* Properly format message */
$content = str_replace(array('<strong>', '</strong>'), array('*', '*'), $content);
$content = str_replace(array('<em>', '</em>'), array('_', '_'), $content);
$content = str_replace(array('<del>', '</del>'), array('~', '~'), $content);
$content = str_replace(array('<li>', '</li>'), array('•', ''), $content);
$content = str_replace(array('<code>', '</code>'), array('`', '`'), $content);
$content = str_replace(array('<pre>', '</pre>'), array('```', '```'), $content);

?>

Hey, thank you for your code, I add handling html links to slack and brs into \n.

        $content = strip_tags($content, '<br><strong><em><del><li><code><pre><a></a>');
        $content = str_replace(array('<br />', '<br>'), "\n", $content);
        $content = str_replace(array('<strong>', '</strong>'), array('*', '*'), $content);
        $content = str_replace(array('<em>', '</em>'), array('_', '_'), $content);
        $content = str_replace(array('<del>', '</del>'), array('~', '~'), $content);
        $content = str_replace(array('<li>', '</li>'), array('•', ''), $content);
        $content = str_replace(array('<code>', '</code>'), array('`', '`'), $content);
        $content = str_replace(array('<pre>', '</pre>'), array('```', '```'), $content);

        preg_match_all('/<a href=\"(.*?)\">(.*?)<\/a>/i', $content, $res);
        for($i = 0; $i < count($res[0]); $i++) {
            $content = str_replace($res[0][$i], '<'.$res[1][$i].'|'.$res[2][$i].'>', $content);
        }

Enjoy ;)

Thank you dami95 for your contribution. I have updated your preg_match_all for links to support additional params before and after the href param. I also moved the linkCount out of the for loop so it follows PHP 8 standards.

        /* Strip out un-supported HTML tags */
        $content = strip_tags($content, '<br><strong><em><del><li><code><pre><a></a>');
        
        /* Properly format message */
        $content = str_replace(array('<br />', '<br>'), "\n", $content);
        $content = str_replace(array('<strong>', '</strong>'), array('*', '*'), $content);
        $content = str_replace(array('<em>', '</em>'), array('_', '_'), $content);
        $content = str_replace(array('<del>', '</del>'), array('~', '~'), $content);
        $content = str_replace(array('<li>', '</li>'), array('•', ''), $content);
        $content = str_replace(array('<code>', '</code>'), array('`', '`'), $content);
        $content = str_replace(array('<pre>', '</pre>'), array('```', '```'), $content);

        /* Process Links and convert to markdown style */
        preg_match_all('/<a.*href=\"([^"]*?)\".*>([^"]*?)<\/a>/i', $content, $res);
        $linkCount = count($res[0]);
        for ($i = 0; $i < $linkCount; $i++) {
            $content = str_replace($res[0][$i], '<'.$res[1][$i].'|'.$res[2][$i].'>', $content);
        }